(a) Field of the Invention
The present invention relates to a light emitting display and a driving method thereof. More specifically, the present invention relates to an organic EL (electroluminescent) display.
(b) Description of the Related Art
In general, an organic EL display electrically excites a phosphorous organic compound to emit light, and it voltage- or current-drives N×M organic emitting cells to display images. As shown in FIG. 1, the organic emitting cell includes an anode (ITO), an organic thin film, and a cathode layer (metal). The organic thin film has a multi-layer structure including an EML (emitting layer), an ETL (electron transport layer), and an HTL (hole transport layer) for maintaining balance between electrons and holes and improving emitting efficiencies. The organic thin film further includes an EIL (electron injecting layer) and an HIL (hole injecting layer).
Methods for driving the organic emitting cells include a passive matrix method, and an active matrix method using TFTs (thin film transistors) or MOSFETs. In the passive matrix method, cathodes and anodes that cross over each other are formed and used to selectively drive lines. In the active matrix method, a TFT and a capacitor are connected with each ITO (indium tin oxide) pixel electrode to thereby maintain a predetermined voltage according to capacitance. The active matrix method is classified as either a voltage programming method or a current programming method based on signal forms supplied to maintain the voltage at the capacitor.
FIG. 2 shows a conventional voltage programming-type pixel circuit for driving an organic EL element (OLED), representing one of n×m pixels.
A transistor Ma coupled between the power supply voltage VDD and an OLED controls the current flowing to the OLED. A transistor Mb transmits a data line voltage to a gate of the transistor Ma in response to a select signal applied from a scan line Sn. A capacitor Cst coupled between a source and the gate of the transistor Ma is charged with the data voltage and maintains the charged state for a predetermined time.
In detail, when the transistor Mb is turned on in response to a select signal applied to the gate of the switching transistor Mb, a data voltage from the data line Dm is applied to the gate of the transistor Ma. Accordingly, the current IOLED corresponding to a voltage VGS charged by the capacitor Cst between the gate and the source of the transistor Ma flows through the transistor Ma, and the OLED emits light corresponding to the current IOLED.
By way of example, the current that flows to the OLED is given in Equation 1.
                              I          OLED                =                                            β              2                        ⁢                                          (                                                      V                    GS                                    -                                      V                    TH                                                  )                            2                                =                                    β              2                        ⁢                                          (                                                      V                    DD                                    -                                      V                    DATA                                    -                                                                                V                      TH                                                                                          )                            2                                                          Equation        ⁢                                  ⁢        1            
where IOLED is the current flowing to the OLED, VGS is a voltage between the source and the gate of the transistor Ma, VTH is a threshold voltage at the transistor Ma, β is a constant, and VDD is a power supply voltage for a pixel.
As given in Equation 1, the current corresponding to the applied data voltage is supplied to the OLED, and the OLED gives light corresponding to the supplied current, according to the pixel circuit of FIG. 2. In this instance, the applied data voltage has multi-stage values within a predetermined range so as to represent gray.
However, when a voltage drop (IR-drop) is generated on a line for supplying the power supply voltage VDD, and the power supply voltage VDD applied to a plurality of pixel circuits is not uniform, a desired amount of current may not flow to the OLED, thereby degrading image qualities, since the current flowing to the OLED is influenced by the power supply voltage VDD in the conventional pixel circuit based on the voltage programming method. As the area of the organic EL display becomes larger, and the brightness increases, the voltage drop on the line for supplying the power supply voltage VDD increases to generate further problems.